The biotechnology industry has evolved to allow the large scale production of recombinant proteins for commercial use. However, many of these therapeutic proteins and vaccines present a unique challenge for drug delivery. The successful clinical application or commercial use of many proteins requires the use of a protein delivery system to reduce the frequency of administration, provide a lower toxicity through a reduced peak serum concentration, localize the drug at the site of action, or yield a steady-state level of the drug to achieve the desired effect. The biggest challenge however is to maintain the stability of proteins during the formulation steps.
Proteins are generally not very stable, as the stabilization energy of the native state is mostly between 5 and 20 kcal/mol, which is equivalent to that of a few hydrogen bonds. Many forces are involved in keeping the native proteins properly folded including hydrophobic interactions, electrostatic interactions (charge repulsion and ion pairing), hydrogen bonding, intrinsic propensities, and van der Waals forces. Among these forces, hydrophobic interactions seem to be the dominant.
There are many factors that affect protein stability. These include at least temperature, pH, ionic strength, metal ions, surface adsorption, shearing, shaking, additives, solvents, protein concentration, purity, morphism, pressure, freeze-thawing and drying. Most mesophilic proteins, such as those from human beings, can be denatured easily at temperatures between 50 to 80° C. A common phenomenon of protein instability is the formation of protein aggregates, which can be soluble or insoluble, chemical or physical, and reversible or irreversible. Chemical transformations that lead to protein instability include at least deamidation, oxidation, hydrolysis, isomerization, succinimidation, disulfide bond formation and breakage, non-disulfide cross linking and deglycosilation. It is therefore clear that developing a delivery system for proteins is not a trivial matter.
The most common approach used to develop a controlled delivery system for proteins is to encapsulate the protein in a polymer matrix or microsphere of poly(lactic-co-glycolic acid) (PLGA), which has been used for over twenty years as a resorbable suture material. A great deal of research has been published on the development of protein-controlled release formulations using PLGA microspheres. However, there are only a small number of successful commercial formulations using this system. A biodegradable microsphere formulation, the Lupron Depot, has been commercially successful for several years. This formulation consists of leuprolide acetate, a decapeptide, encapsulated in biodegradable microspheres of PLGA for the treatment of prostate cancer, endometriosis, and precocious puberty. The drug is released continuously over either one or three months depending upon the formulation. A biodegradable human growth hormone microsphere formulation is presently marketed by Alkermes, Inc.
PLGA microspheres are usually prepared by a solvent-based process, which as discussed above, is harmful to protein stability and activity. Melt processing is not a viable alternative, because the Tg of the PLGA system is in the range of 55 to 60° C. Thus, melt processing has to be conducted at relatively high temperatures that are detrimental to the stability and activity of the proteins. PLGA degradation also releases acidic byproducts, which can inactivate the proteins.
Drug delivery systems require a product that is injectable through a 21 gauge needle and from which a protein can be delivered at a sustained rate for two to four weeks without loss of activity. Water may be used as a diluent to allow injection through a narrow bore needle, and stabilizers such as sugars or salts may be used.
Accordingly, there exists a need for a biocompatible low viscosity polymer that is degradable and resorbable and able to absorb water or be emulsified in water so that water can be used as a diluent, if necessary. Ideally, for drug delivery purposes, the polymer should be fluid at room temperature. However, polymers that become fluid with heating to temperature below the temperature at which proteins denature are also suitable.